Horizontal directional drilling is a process commonly utilized to create boreholes for the installation of utilities underground. The process involves a drilling machine, a drill string and a drill head. The drill string is typically composed of individual sections of hollow drill rod, and is attached above ground between the drilling machine and the drill head. The drilling machine is typically capable of rotating and longitudinally propelling and thrusting the drill string, while simultaneously pumping a fluid through the drill string. The drill head is typically composed of an adapter assembly and a drill bit. There are many types of adapter assemblies, including static and dynamic, each typically connecting on one end to the drill string, and on the other end to the drill bit. There are a variety of drill bits, each designed to be used in conjunction with a specific type of adapter.
The process starts with installing the drill head onto a single drill rod above ground. The drill rod is then connected, at the opposite end, to a drilling machine. The drilling machine then rotates and pushes the drill rod and drill head into the ground. At the same time, a fluid is pumped through the drill rod and typically directed to the cutting surface of the drill bit to assist in cutting the ground material.
The pumped fluid has a variety of purposes. One primary purpose relates to removal of material to create the borehole. In this application, fluid transports cuttings created by the drill bit back along the bored hole and out to the ground surface. In most setups, a particular drill bit is configured to cut a hole larger than the drill rod diameter by disturbing the soil formation as it is rotated. Examples of such bits can be found in U.S. Pat. Nos. 5,799,740 and 5,899,283. At the same time, a water-based fluid is pumped through the drill string and through the bit to thoroughly mix with the disturbed soil, creating a slurry. The slurry then follows the path of least resistance, which is typically back along the drill string, and exits at the point the drill string enters the ground. In this application the adapter assembly is static, simply adapting from the drill rod threaded connection, which is smaller diameter, to the drill bit, which is larger in diameter to cut the larger hole required for the proper transfer of cuttings.
In some other applications there is no requirement to transport the cuttings and the ground is simply compacted, forming a borehole without any material removal. Impact or hammering load on the drill bit increases the productivity of drilling. For this type of application, the adapter assembly includes a dynamic component, typically a pneumatic hammer, in addition to a static adapting element. (An example disclosed in U.S. Pat. No. 4,858,704.) The fluid being pumped in the drill string is compressed air that transfers power to actuate the pneumatic hammer. The path of fluid flow includes the drill string, the static component of the adapter assembly, and the hammer.
In yet other applications, typically highly compressed soils and or rock, a similar setup utilizing a down hole hammer can be used in conjunction with a different drill bit to create cuttings for transport. The hammers can be pneumatic hammers or water hammers. The drill bits are designed primarily to fracture the soil or rock formation by the impact loading received from the hammer. Once the formation is fractured, the cuttings need to be transported away from the cutting face.
Transportation of the cuttings is aided by rotation of the drill bit and drill string, along with the resulting flow of the fluid. The fluid is typically air or a mixture of air and a water based fluid or other suspension material which functions to aid the air's ability to transport the cuttings. In this type of application, the fluid is utilized to transfer power to actuate a hammer to transport cuttings. The path of fluid flow includes the drill string, adapter assembly and drill bit.
In still another arrangement involving cutting highly compressed soils or rock, the drill bit is adapted to rotate. One such design includes the use of a mud motor capable of converting fluid power (from the pumped fluid) into rotational power to rotate the drill bit. In this type of application, the adapter assembly includes a dynamic component, the mud motor, along with the previously described static element. The fluid is typically water based. The path of fluid flow includes the drill string, the adapter assembly and the drill bit.
In all these applications, the transfer of fluid assists in the efficient functioning of the drill bit and/or transportation of the cuttings; relatively large flow rates may be required. The path of fluid flow, in all cases, is through the adapter. Thus a key characteristic of the adapter is fluid transfer capability.
Another key aspect of horizontal directional drilling is the detection of location and position of the drill head. This information is necessary to properly control the drilling process so that the bored hole is properly positioned. This is typically accomplished by installing tracking electronics in the drill head, typically in the form of a sonde. Sondes are currently available in a variety of sizes, from a variety of manufacturers and include 2 basic types; a type powered by a battery and a type powered by a wire that is threaded through the drill string to an above-ground power source.
An example of a battery powered sonde and its mounting configuration within a drill head is described in U.S. Pat. No. 5,633,589. FIG. 4 of '589 illustrates a drill head with the adapter assembly connected on one end to the drill string and to the drill bit at the other end. This is a schematic representation illustrating primarily the electronic package. This arrangement illustrates that the adapter assembly is configured to hold the sonde or transmitter which is generally cylindrical and whose diameter is significant in relation to the diameter of the drill rod. This static section of the adapter assembly has become known as the sonde housing.
Other examples of sonde housings can be seen in U.S. Pat. No. 5,799,740 (hereinafter '740), U.S. Pat. No. 5,253,721 (hereinafter '721), and U.S. Pat. No. 6,260,634 (hereinafter '634). FIG. 11 of '740 more closely exemplifies the design of typical sonde housings. The housing is configured to accept a sonde, to mate to a drill bit, to mate to the drill string, and to provide a passage for fluid. The mechanical configuration is such that a cavity for the sonde is positioned off center and located as close as possible to the edge of the adapter, as constrained by minimum material thickness. This provides a maximum cross-sectional area of the fluid passages, also constrained by minimum material thickness surrounding the passage. The location of the fluid passages is thus close to the outer diameter of the sonde housing.
In order to manufacture typical sonde housing passages, the sonde housing is made as two pieces. The cylindrical main section, illustrated as FIG. 11 in '740, includes a threaded section with an inner diameter sufficiently large to allow the fluid passages to be manufactured with normal drilling. This thread is much larger than the threads utilized on the drill rod. Thus a second piece, illustrated in FIG. 10, screws into these large threads on one end and adapts to the threads of the drill string on the other end. In this manner, the sonde housing is constructed from multiple parts that are screwed together. The sonde is installed into the sonde housing by separating the two pieces at this threaded connection. This type of sonde housing is referred to as an end load sonde housing as the sonde is inserted from an end of the sonde housing.
The cylindrical sonde housing illustrated in the '634 patent also utilizes a two piece construction. FIG. 2 illustrates a similar main section adapted to accept a sonde, adapted to a drill bit on one end, and to a second adapter on the opposite end. Rather than utilizing a threaded connection between the main section and the adapter, this sonde housing utilizes a splined connection. One such adapter is illustrated in FIG. 22 of U.S. Pat. No. 6,148,935 (hereinafter '935), and herein incorporated in its entirety by reference. Here again, the inner diameter of the splined connection is such that the fluid transfer holes can be drilled with normal drilling techniques. The sonde housing illustrated in the '634 patent is generally referred to as a side load housing as the sonde housing includes a door that covers the sonde cavity mounted on the side of the sonde housing and the sonde is accessed from the side.
FIG. 1 of '935 and FIG. 3 of '721 illustrate the difficulty of manufacturing a one-piece sonde housing. In '935 the fluid transfer holes are drilled at an angle, adding cost and complexity to the assembly. In '721 the fluid transfer holes require 4 separate, intersecting drilled holes creating 90-degree angles in the fluid pathway. This configuration results in significant flow restriction.
In addition to providing a flow passage, the sonde housing also serves to support and position the sonde. U.S. Pat. Nos. 6,260,634 and 6,148,935 illustrate the use of a splined connection between the sonde housing and the drill bit that can only be assembled in one rotary orientation. This, combined with the rotary orientation control of the sonde, coordinates the orientation between the sonde and the drill bit. This arrangement is dependent on the splined connection, which results in restricting the variety of drill bits that can be utilized with the housing, as not all bits include such splines.
Other mounting requirements for sondes include vibration isolation, particularly when the adapter assembly includes a hammer, and/or provision for a wire passage for use with a wire-line sonde. The sonde housing, being located near the drill bit, is subjected to severe load conditions. The mechanical rigidity and assembly characteristics affect the durability of the sonde housing. The requirement for durability is exemplified by the existence of industry standards for certain types of drilling components. For instance, the American Petroleum Institute has adopted a specific thread configuration for use with drilling components that imposes an additional physical constraint affecting the mechanical configuration of the sonde housing.